Electrodynamic loudspeakers, especially those intended to be of low cost as for utilization in automobiles and the like, typically use small volume or low weight magnets for the diaphragm motor, resulting in a low damping factor on the moving system, quantitatively defined by "Q", resulting in a "Q" in excess of approximately 1.2. This low damping factor has a deleterious effect on the acoustical performance in the vicinity of the principal (lowest) resonant frequency of the moving system characterized by a peak in the steady state acoustical output with a concomitant increase in harmonic and intermodulation distortion and impaired transient performance resulting in "ringing" of the system. The same low cost speakers with small magnets also typically utilize sound radiating diaphragms commonly called cones, having a low mass characterized by a weight to radiating area ratio typically in the 0.04-0.15 gm/in..sup.2 range, in order to maximize the sensitivity. The low mass cone also tends to increase the amplitude of distributed mode resonances in the cone, which results in an increased sound output in the upper frequency range, i.e. above about 2,000 Hz, and which may not be desirable. A further performance problem in low cost, small size, low cone-mass loudspeakers is that the principal resonant frequency, f.sub.0, which establishes the low frequency limit of performance in many applications, cannot be made as low as desired due to cone manufacturing limitations involved in felting the outer cone suspension areas sufficiently thin. The principal resonant frequency for a given mass cone is a function of the cone's suspension compliance which in turn is a function of the thickness of the suspension area of the cone.
The increased levels of sound output in the vicinity of the principal resonance frequency and in the upper frequency range of the loudspeaker are not always desirable performance attributes. A uniform or "flat" amplitude vs. frequency characteristic is often desired but difficult to achieve.
One technique for reducing the amplitude of the peak in the sound output in the vicinity of the principal resonant frequency is the use of an acoustic resistance covering the openings in the frame of the loudspeaker on the rear side of the cone, as discussed in an article entitled "Acoustic Resistance Damping for Loudspeakers" by John L. Grauer in AUDIO, Vol. 49, No. 3, p. 22, March 1965. The increased damping is accomplished by the increased resistance encountered by the volume flow of air set in motion by the cone as it passes through the acoustical resistance material. This approach is generally effective in attenuating the resonant peak in the response, however, it does not offer a means for reducing the level of the upper frequency response or for beneficially lowering the principal resonant frequency. The Grauer article considers the possibility of applying resistive damping in front of the cone to filter acoustically the highest frequencies, where that is desirable, however, no discussion of the geometry and characteristics of that latter type of damping nor of a means for concomitantly lowering the principal resonant frequency of the loudspeaker is included.
In U.S. Pat. No. 2,840,178 entitled "Device for the Reproduction of Sound", the provision of an acoustical impedance for loudspeakers positioned in front of the loudspeaker cone is discussed. The acoustic impedance consists of an acoustic resistance and an acoustic mass. The resistance preferably "consists of a rigid or reinforced disc of wire-net, or a perforated metal sheet on which there is cemented a material acting as acoustic resistance". An opening in the center of the disc acts as acoustic mass. In an alternative embodiment, the acoustic mass is constituted by an auxiliary diaphragm or cone disposed in front of the low frequency diaphragm of the loudspeaker. That auxiliary diaphragm may either be provided with openings which are filled or covered with a material acting as acoustic resistance, or it may be made entirely of an air permeable material which acts as an acoustic resistance, but in either instance appears to take the form of a conical diaphragm. No further details are provided concerning the characteristics of the material.
A further patent, U.S. Pat. No. 4,012,605 entitled "Input/Output Transducer with Damping Arrangement", provides a grill in front of a speaker/microphone cone, and some of the interstices within the grill contain segments of a damping material. Although the damping material is said to improve the frequency response, that improvement would appear to be limited to use in the microphone mode, inasmuch as the speaker response curve 26 of FIG. 7 continues to show a relatively significant peak.
It is a principal object of the present invention to provide an improved loudspeaker utilizing a supplemental damping impedance comprised of acoustic resistance and an integral acoustic mass element operative on the frontal radiation of a loudspeaker. Included within this object is the provision of acoustic impedance especially suited for use with loudspeakers in automotive applications requiring relative immunity to moisture. Also included within this object is the provision of such acoustic impedance with a low cost speaker normally having a Q greater than about 1.2.
In accordance with the invention, there is provided in a direct radiator dynamic loudspeaker having a natural Q greater than about 1.2 an improved acoustic impedance. The acoustic impedance element is substantially planar and is positioned in front of and covers the projected frontal radiation area of the diaphragm or cone. The acoustic impedance element is supported only about its periphery and comprises a fibrous felt material having an airflow resistance in the range of about 50-100 cu. ft. per min. at 0.5 psi pressure drop, a density in the range of about 6-12 oz. per sq. yard and a thickness in the range of 0.03-0.09 in. The acoustic impedance element is in an air permeable continuous sheet and is comprised substantially entirely of fibers of synthetic materials, as for instance polyester, which are relatively impervious to moisture. The Q of the speaker including such damping element is less than about 1.2, being about 0.75-1.0.